fix: prevent illegal negative timeout values into thread sleep() method while retrying exceptions in unit tests. (#2268)

* fix: prevent illegal negative timeout values into thread sleep() method while retrying exceptions in unit tests. * For details on issue see - #2206

Author: arpan14

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITClosedSessionTest.java

@@ -251,7 +251,10 @@ public void testTransactionManager() throws InterruptedException {
  break;
  }
  } catch (AbortedException e) {
- Thread.sleep(e.getRetryDelayInMillis());
+ long retryDelayInMillis = e.getRetryDelayInMillis();
+ if (retryDelayInMillis > 0) {
+ Thread.sleep(retryDelayInMillis);
+ }
  txn = manager.resetForRetry();
  }
  }